We will use a single does of 44 Calcium given orally and a single dose of 46 Calcium given intravenously to measure: 1) true calcium absorption, 2) endogenous fecal excretion, 3) urine calcium excretion, 4) exchangeable calcium pool size, 5) total calcium retention and 6) bone calcium excretion. These calculations will be made from measurements of 44 Ca and 46 Ca using thermal ionization mass spectrometry in six four hour urine pools, a single marked stool, and a single serum sample. This technique will be used for studying premature infants because of its safety and ease. This technique will be used to study the effects of: 1) vitamin D (high dose vitamin D, 25-OHD3 and 1,25(OH)2D), 2) mineral intakes (calcium and phosphorus), 3) gestational age, 4) postnatal and/or post-conceptional age and 5) other nutritional components of milk e.g. protein on the stated calcium dynamics. Data from the stable isotope studies will be correlated with other measurements of mineral homeostasis and mineralization: Serum vitamin D, 25-OHD, and 1,25(OH)2D, PTH, calcitonin, serum calcium, phosphorus, magnesium, albumin, and alkaline phosphatase, urine minerals, and bone mineral content by photonabsorptiometery. This characterization of calcium dynamics in the premature infant and the factors which effect calcium absorption and retention, will be used to design special cow milk based formulae for premature infants as well as supplements for human milk that will result in optimal growth, mineral homeostasis, and bone mineralization. With infants less than 1500g, and even less than 1000g, surviving in increasing numbers it is imperative that systems to adequately nourish these infants be developed.